Note 9 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Notes Tables

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block]

	Year Ended December 31,
	2016		2015		2014
Research and development	$	1,114	$	650	$	114
General and administrative		2,229		1,514		345
Total	$	3,343	$	2,164	$	459

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Year Ended December 31,
	2016			2015			2014
Weighted average expected volatility		84.4	%		79.8	%		79.5	%
Expected term (in years)		6.05			6.11			6.00
Risk free interest rate		1.45	%		1.76	%		1.88	%
Expected dividend yield		0	%		0	%		0	%

Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]

	Options			Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term (years)		Aggregate Intrinsic Value
Outstanding at December 31, 2015		2,200,369		$	8.52		7.8	$	15,705
Granted		132,495		$	7.11
Exercised		(97,521	)	$	2.01
Cancelled or forfeited		(69,089	)	$	14.12
Outstanding at December 31, 2016		2,166,254		$	8.55		7.1	$	124
Exercisable at December 31, 2016		1,227,575		$	6.80		6.0	$	124
Vested or expected to vest at December 31, 2016 (1)		2,052,214		$	8.37		7.0	$	124